ELECTRICITY. 83 



the base be a metal, such as copper, it will be deposited on the elec- 

 tronegative pole. Use is made of this property in the different 

 processes of electroplating and electrotyping. Among the metals 

 requiring the weakest current for their electrolytic precipitation are 

 copper, silver, gold, and nickel, and these are often precipitated upon 

 other metals which form the negative pole. 



Electrolysis is used also on a large scale for separating metals from 

 ores, or from one another, and on a small scale for analytical opera- 

 tions ; it also plays an important part in the work done by the elec- 

 tric furnace, in which often both the required high temperature and 

 the decomposing influence are furnished by the electric current. 



Discharge through gases. When an electric current of suffi- 

 ciently high E. M. F. is discharged through a gas, for instance, 

 through air, the gas is rendered luminous by its passage i. e., we 

 have what is called an electric spark, in appearance like that of a flash 

 of lightning. This spark in many cases exerts chemical action, as, 

 for instance, when oxygen is converted into ozone, which change will 

 be considered later. 



If the discharge take place in a gas inclosed in a glass globe from 

 which the gas may be removed by means of an air-pump, it is found 

 that at a sufficiently reduced pressure the spark ceases and the inte- 

 rior of the globe assumes a beautiful luminosity, the nature of which 

 depends on the kind of gas operated on. 



By exhausting the air from suitably constructed bulbs or tubes 

 still further until an almost perfect vacuum is obtained, the electric 

 discharges passing through this vacuum again show decided changes. 

 If the cathode of such a vacuum tube be formed of a metallic disk, 

 while the anode be a straight wire, then on passing the current 

 through the vacuum tube a pale purplish beam of light radiates from 

 the face of the disk. This beam is known as the cathode ray ; it has 

 been shown to consist of streams of portions of matter negatively 

 charged, and moving with great velocity. 



Rontgen rays. During the generation of the cathode rays in the 

 vacuum tube another kind of rays of a very peculiar character are formed. 

 They have been called Rontgen rays, after their discoverer, who him- 

 self named them x-rays. These rays differ from other rays in many 

 respects ; thus they can pass readily through many kinds of matter 

 which are opaque to ordinary light ; they cause many substances tg 

 emit light when they fall upon them ; they affect photographic plates, 



